Micro-machined thermal infrared (IR) detectors are a well-established technology, and are typically based on thermopiles, bolometers, pyrodetectors or even diodes. These typically include of a structure thermally insulated from the substrate (such as a membrane or micro-bridge) which heats up due to incident IR radiation, and this change in temperature is detected using various methods. For example Graf et al. “Review of micromachined thermopiles for infrared detection” Meas. Sci. Technol. 18 (2007) R59-R75 reviews several thermopile based IR detectors which are typically on a membrane. Kim et al., “A new uncooled thermal infrared detector using silicon diode,” Sensors and Actuators A 89 (2001), pp. 22-27, discusses a diode on a micro-bridge membrane used for IR radiation measurement.
It is also well known to fabricate arrays of IR detectors. For example, Hirota et al., “120×90 Element thermopile array fabricated with CMOS technology,” Proceedings of SPIE Vol. 4820 (2003) pp. 239-249 describe an array of thermopile IR detectors, where each IR detector pixel is a separate front-side etched membrane.
Sarro et al., “An integrated thermal infrared sensing array,” Sensors and Actuators 14 (1998) pp. 191-201, describe a linear 8-element thermopile array where each IR detector is on a cantilever structure. Jones et al., “MEMS thermal imager with optical readout”, Sensors and actuators A 155 (2009), pp. 47-57, describe a two dimensional array where each detector is on a cantilever.
Foote et al., “High performance micromachined thermopile linear arrays,” SPIE Vol. 3379, 1998, pp. 192-197, describe a linear array with each thermopile IR detector on a micro-bridge.
Calaza et al., “An uncooled infrared focal plane array for low-cost applications fabricated with standard CMOS technology,” Sensors and Actuators A 132 (2006) pp. 129-138, describe a two dimensional IR detector array, where each detector is on a suspended membrane/microbridge structure.
Kanno et al., “Uncooled infrared focal plane array having 128×128 thermopile detector elements,” SPIE Vol. 2269, pp. 450-459 describe a 128×128 IR detector array, where each element is on a suspended membrane/diaphragm.
U.S. Pat. No. 7,842,922 describes an IR detector array based on thermopiles, where each element is on membrane.
U.S. Pat. No. 7,005,644 describes an IR detector array based on diodes, where each element is on a separate micro-bridge type structure.
In each of these examples, the individual element of the array is on its own separate membrane/cantilever/diaphragm structure. These structures usually cannot be made very small, and as a result the pixel size tends to be large. This results in the overall array occupying a large chip area and is costly.
However, a number of linear arrays with 1 or 2 rows are known in literature. For example:
Baer et al., “A 32-element micromachined thermal imager with on-chip multiplexing” Sensors and Actuators A 48 (1995) pp. 47-54 described a 2×16 IR detector array on a single membrane.
Kessler et al., “A 256 pixel linear thermopile array using materials with high thermoelectric efficiency,” Proceedings of Thermoelectrics 1997, pp. 734-737, and Dillner et al., “A 64-pixel linear thermopile array chip designed for vacuum environment,” Proceedings of IRS2 2006, pp. 295-300, both describe linear arrays.
EP1413861B1 describes a linear array with only one row, having all the detectors on a single membrane.
U.S. Pat. No. 8,441,093 describes linear arrays of detectors based on thermopiles all on the same membrane, where the cold junction is on the substrate. It also shows a circular membrane with thermopiles placed radially, with the cold junctions on the substrate. In this case as well, the array cannot have a large number of pixels as cold junction connection to the substrate is needed.
These structures are possible as with a 1 or 2 row array of IR detectors on the same membrane, there is a substrate region next to each pixel which is cooler than the pixel area. For example, if using a thermopile based detector, the substrate region can be used as the cold junction region. However, for an array including 3 or more rows, the rows in the centre do not have access to the substrate region. For example, EP1413861 describes an array of detectors including of several rows, but they are made with each two row on a single membrane to allow a cold junction on the substrate for the thermopile.
In the above documents, it still requires several membranes for a large number of rows.
U.S. Pat. No. 6,040,579 and Schaufelbuehl et al., “256-Pixel CMOS-Integrated Thermoelectric Infrared Sensor Array” Proceedings of MEMS 2001, pp. 200-203, describe an array of thermopile IR detectors all on the same membrane, and use gold on top of the membrane to separate each pixel and act as the heat sink.
Similarly, Oliver et al., “A 1024-element bulk-micromachined thermopile infrared imaging array,” Sensors and Actuators 73 (1999) pp. 222-231, describe a two-dimensional array on a single membrane, where the device thermal isolation is performed by the use of thick silicon underneath the membrane.
Rubio et al., “Thermopile sensor array for an electronic nose integrated non-selective NDIR gas detection System,” Proceedings of Spanish Conference on Electron Devices 2005, pp. 503-505, describe a two dimensional IR detector array on a single membrane, using silicon below the membrane to provide the cold junction for the thermopiles.
In these cases a single membrane is used for the entire array. However, in case of using silicon underneath the membrane as a heatsink, the etching process is much more complicated. Additionally, the silicon heat sinks cannot be made very small, hence the spacing between pixels is large.
In the case of using gold tracks on top of the membrane, gold is not CMOS compatible, and this results in a non-standard process and increased fabrication cost.